Arrow weight and poundage

I am kinda new to shooting and was reading different threads and seen that people talk about 5gpi on arrows. I shoot a bow that pulls 68 pounds and shoot a arrow that weighs 425 grans with everything on them. Now if i am thinking right with what i have read i can go down to a arrow that weighs a total of 340 grans and it would be alright to shot in my bow. Is this right? And if this is right how much speed would i gain by droping that much weight? Right now i am shooting 287 in my bow. Any info on this would be appreciated.

I was looking at some of the arrows and found a carbon express line jammers-ss i need them 31 inchs they weights is 7.6 grans i thought with these arrows and 100 gran tips and blazer vanes if i did it right they will weight 341 grans. Will this work with 68 pound pull?

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